1. Field of the Invention
This invention relates to a wafer holding assembly for holding a wafer in an ion implantation apparatus used when ions are implanted into a wafer such as a silicon wafer, and more particularly to a wafer holding assembly capable of controlling a temperature distribution in a plane of the wafer.
2. Description of the Related Art
At this moment, a typical production method of an SIMOX wafer is called an MLD (Modified Low Dose) method, wherein an oxygen ion implantation is conducted in two stages (see Patent Document 1). That is, the first oxygen ion implantation is carried out by heating a silicon wafer to a higher temperature, and then the second oxygen ion implantation is carried out by lowering the temperature of the silicon wafer to about room temperature. In the first oxygen ion implantation, the silicon wafer is heated to form a layer having a high oxygen concentration on the surface of the silicon wafer while maintaining at a state of a single crystal, and in the subsequent second oxygen ion implantation is formed an amorphous layer. Thereafter, the high-temperature heat treatment is conducted in a mixed gas of oxygen and argon to form an SOI structure.
[Patent Document 1] U.S. Pat. No. 5,930,643
In an oxygen ion implantation apparatus used for producing the above SIMOX wafer, a holding assembly 1 as shown in FIG. 1 is known as a wafer holding assembly (see Patent Document 2). The wafer holding assembly 1 has a mechanism as shown in FIG. 1 that wafer holding pins 2 are fixed to three end places of a holder 10 made of a substantially Y-shaped frame body and a wafer 3 is held at the three places of the holder through these holding pins 2.
[Patent Document 2] U.S. Pat. No. 6,794,662
As shown in FIGS. 2A-2C, the holding pin 2 comprises a head 4 contacting with an end face of the wafer 3 to control the motion of the wafer 3, and a flange 5 projecting from the head 4 to place the wafer 3. As shown in FIG. 2C, the wafer 3 is held at a state of being sandwiched between the head 4 and the flange 5.
In the conventional wafer holding assembly 1, the head 4 is made small and the flange 5 is formed to have a diameter larger than that of the head 4 in order to pervade irradiation beams all over the end portion of the wafer at the oxygen ion implantation step, for example, in the production of SIMOX wafer or to prevent so-called shadowing. For example, when the form of the holding pin 2 is a typical cylindrical form, the diameter of the flange 5 is commonly not less than 1.5 times the diameter of the head 4.
In the production method of the SIMOX wafer through the above MLD method, however, when the temperature of the wafer is particularly raised by the irradiation of beams in the second oxygen ion implantation, there is caused a temperature difference between the wafer and the holding pin because they are different in the shape and material. The influence of such a temperature difference appears as a thickness irregularity of an amorphous layer in the wafer located in the vicinity of the holding pin. In the wafer after the high-temperature heat treatment, therefore, one unresolved problem is thickness irregularity, particularly in SOI or BOX layers in the vicinity of the holding pin 2.